(1) Field of the Invention
This invention relates to analog to digital converters (ADC), and more particular to a wideband analog to information converter using wavelet filters, a feature extracto, and digital signal processing algorithms.
(2) Description of Related Art
The prior art FIG. 1 illustrates a typical hybrid analog/digital filter bank ADC, where an analog analysis filter bank 100 with filters 102 splits the input signal 104 into M uniform frequency bands. The circuit includes a Radio Frequency (RF) receiver antenna 101 for receiving incoming signals, and a unit 107 for removing the carrier portion of the incoming signal. Unit 107 is comprised of a low noise amplifier 103 that amplifies the incoming signal, an oscillator 105 that modulates the signal to differentiate between the carrier signal and the information containing portion, and a filter 109 that actually filters out or removes the carrier.
The filters 102 of prior art FIG. 1 are characterized by their transfer functions H such as Butterworth or Chebyshev that basically describe a particular method of filtering. By splitting the input signal 104, the effective bandwidth of the signals incident to each sample-and-hold unit 106 is reduced by a factor of M and, hence, the sampling frequency of each converter can be reduced by a factor of M without loss of information with respect to frequency. As illustrated, the input filtered signals are sampled by sample-and-hold units 106, and then processed by the channel processors 108 to the desired resolution of the converter. The channel processors 108 function as quantizers to convert the incoming sampled signal to a binary representation, and also function to remove any unwanted effects (distortions) in an incoming signal 104 due to the medium (e.g. air) through which the signal 104 traveled. In order to achieve a higher data rate, each channel is up sampled by a sample-and-hold unit 110 by a factor of M (because the signals were down-sampled by a factor of M by down samplers 106). The signals are then reconstructed through a digital synthesis filter bank 114 with digital filters 116, and combined at 118 to output a reconstructed signal 120 in the digital domain. The transfer functions of the digital filters 116 are matched with the transfer functions of filters 102 for recombination and reconstruction of the signal at 118. The hybrid filter banks use uniform samplers (sample-and-hold) 106, channel processor 108 and up samplers 110.
The illustrated prior art system functions as a channelized receiver based on the short-time Fourier Transform of signals, and as illustrated in FIGS. 2A and 2B, has no fidelity in time-localized signals. The frequency localized part of the signal in FIG. 2A is clearly represented as the Essential Fourier Coefficients in FIG. 2B, but the time localized part of the input signal illustrated in FIG. 2A is missing from its frequency representation, as illustrated in FIG. 2B. The fundamental basis for this problem is the existing trade-off between frequency and time discrimination of an incoming signal, which is an inherent limitation caused by the existing inverse relationship (inverse proportionality) between these two parameters, where frequency=1/time and time=1/frequency. A broad band in terms of frequency is therefore commensurate to a narrow band in terms of time, and vice versa. Accordingly, variations in narrow band of time (short duration), illustrated as the two spikes in FIG. 2A, may only be “captured” by wide bandwidth filters, and variations in the wide band of time (slow changing) are “captured” by narrow bandwidth filters, such as those used in the prior art FIG. 1. As is clearly illustrated in FIGS. 2A and 2B, for signal variations within a short-duration (such as the spikes illustrated in FIG. 2A), the filtering system of FIG. 1 fails because it cannot “see” them, and hence the output signal 120 cannot accurately represent the entire input signal due to this “unseen” portion.
The prior art ADC system illustrated in FIG. 1 digitizes the entire signal 104, with no intelligence to discriminate between the signal portions containing the required or necessary information and those with no information. This is an inefficient use of resources, which assumes that Digital Signal Processes (DSPs) coupled at the output 120 (not shown) can handle the processing load (i.e. clean-up or removal of the already digitized portions of the signal that carry no information). Therefore, in the process of digitizing the entire signal, the ADC resources are not efficiently utilized. In fact, even interference and jammer signals are converted because no effective interference and jammer mitigation strategy is provided.
In light of the current state of the art and the drawbacks to current systems mentioned above, a need exists for a system and a method that would mitigate these problems and would directly convert analog signal to required information. For conversion of a wideband signal to capture signal variations in short duration, there is a need for wideband ADCs that would have high dynamic range, would have low power consumption, and would have high speed and a wide frequency band of operation, which are currently not available.